Name
ARB_shader_stencil_export
Name Strings
GL_ARB_shader_stencil_export
Contributors
Graham Sellers, AMD
Andrew Lewycky, AMD
Mais Alnasser, AMD
Contact
Graham Sellers, AMD (graham.sellers 'at' amd.com)
Notice
Copyright (c) 2010-2013 The Khronos Group Inc. Copyright terms at
http://www.khronos.org/registry/speccopyright.html
Status
Complete. Approved by the ARB on June 9, 2010.
Approved by the Khronos Board of Promoters on July 23, 2010.
Version
Last Modified Date: 05/20/2010
Author Revision: 6
Number
ARB Extension #106
Dependencies
OpenGL 1.0 is required.
ARB_fragment_shader is required.
This extension is written against the OpenGL Shading Language Specification
version 1.40.05
Overview
In OpenGL, the stencil test is a powerful mechanism to selectively discard
fragments based on the content of the stencil buffer. However, facilites
to update the content of the stencil buffer are limited to operations such
as incrementing the existing value, or overwriting with a fixed reference
value.
This extension provides a mechanism whereby a shader may generate the
stencil reference value per invocation. When stencil testing is enabled,
this allows the test to be performed against the value generated in the
shader. When the stencil operation is set to GL_REPLACE, this allows a
value generated in the shader to be written to the stencil buffer directly.
IP Status
No known IP claims.
New Procedures and Functions
None.
New Tokens
None.
Additions to Chapter 2 of the OpenGL 2.1 Specification (OpenGL Operation)
None.
Additions to Chapter 3 of the OpenGL 2.1 Specification (Rasterization)
None.
Additions to Chapter 4 of the OpenGL 2.1 Specification (Per-Fragment Operations
and the Framebuffer)
None.
Additions to Chapter 5 of the OpenGL 2.1 Specification (Special
Functions)
None.
Additions to Chapter 6 of the OpenGL 2.1 Specification (State and
State Requests)
None.
Additions to the OpenGL Shading Language Version 1.40.05
Add a new Section 3.3.x, GL_ARB_shader_stencil_export Extension (p. 11)
3.3.x GL_ARB_shader_stencil_export
To use the GL_ARB_shader_stencil_export extension in a shader it must be
enabled using the #extension directive.
The shading language preprocessor #define GL_ARB_shader_stencil_export will
be defined to 1 if the GL_ARB_shader_stencil_export extension is supported.
Modify Section 7.2, "Fragment Shader Special Variables":
Add to the list of built-in special variables, p.63:
out int gl_FragStencilRefARB;
Modify the paragraph beginning, "Fragment shaders output values to the
OpenGL pipeline..." to:
Fragment shaders output values to the OpenGL pipeline using the built-in
variables gl_FragColor, gl_FragData, gl_FragDepth, and gl_FragStencilRefARB
unless the discard statement is executed. Both gl_FragColor and gl_FragData
are deprecated; the preferred usage is to explicitly declare these outputs
in the fragment shader using the out storage qualifier.
Insert a new paragraph after the paragraph describing gl_FragDepth:
Writing to gl_FragStencilRefARB will establish the stencil reference value
for the fragment being processed. Only the least significant bits of the
integer gl_FragStencilRefARB are considered up to the value of STENCIL_BITS
and higher order bits are discarded. If stencil testing is enabled and no
shader writes to gl_FragStencilRefARB, then the fixed function value for
stencil reference will be used as the fragment's stencil reference value.
If a shader statically assignes a value to gl_FragStencilRefARB, and there
is an execution path through the shader that does not set
gl_FragStencilRefARB, then the value of the fragment's stencil reference
value may be undefined for executions of the shader that take that path.
That is, if the set of linked shaders statically contain a write to
gl_FragStencilRefARB, then it is responsible for always writing it.
Modify the first paragraph on p.64:
If a shader executes the discard keyword, the fragment is discarded, and
the values of any user-defined fragment outputs, gl_FragDepth, gl_FragColor,
gl_FragData, and gl_FragStencilRefARB become irrelevant.
Additions to the AGL/GLX/WGL Specifications
None.
GLX Protocol
None.
Errors
None.
New State
None.
New Implementation Dependent State
None.
Issues
1) Should gl_FragStencilRefARB be initialized to the current stencil
reference value on input to the fragment shader?
RESOLVED: No. gl_FragStencilRefARB is write-only. If the current stencil
reference value is required in a shader, the application should place
it in a uniform.
2) Is it possible to output the stencil mask from the shader?
RESOLVED: No.
3) Is the global stencil mask still applied?
RESOLVED: Yes. The mask is global as there is no way to export the
stencil mask from the shader.
4) How is two-sided stencil handled? How do we export front and back
stencil references?
RESOLVED: There is only one export from the shader. However, two sided
stencil reference generation may be achived as:
if (gl_FrontFacing)
gl_FragStencilRefARB = foo;
else
gl_FragStencilRefARB = bar;
If both front and back stencil reference values are needed in a single
shader, user defined uniforms may be used.
5) If the value written to gl_FragStencilRefARB is outside of the range of
values representable by the stencil buffer, what happens?
RESOLVED: The additional bits are discarded. This is logical as the
stencil mask is still applied and its bit-depth is that of the stencil
buffer. If clamping is desired, this should be performed in the shader.
6) Why is gl_FragStencilRef signed?
For consistency with the ref parameter to glStencilFunc, which is a
GLint. It is masked with the stencil mask value, so it makes no practical
difference whether it's signed or unsigned unless the implementation
supports 32-bit stencil buffers. An interesting note is that the GL
specification states that the comparison made in the stencil test
is unsigned.
Revision History
Rev. Date Author Changes
---- -------- -------- -----------------------------------------
6 05/20/2010 gsellers Add issue 6.
5 04/07/2010 gsellers Remove erroneous language stating that
gl_FragStencilRefARB is initialized to the fixed
function reference value.
4 09/27/2009 gsellers Update issues 1, 4 and 5.
3 07/16/2009 gsellers Address two-sided stencil references.
Add issues 3, 4 and 5.
2 07/14/2009 gsellers Add ARB suffix to gl_FragStencilRef. Resolve (1)
1 07/10/2009 gsellers Initial draft.